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If you’ve been around GPS mapping for any length of time, I’m sure you’ve heard of post-processing, and you may have even experienced it yourself. If you used GPS for mapping in the ’90s, you almost certainly post-processed your data. In fact, sometimes you had to pay for access to GPS base-station data for post-processing. That’s hard to imagine given the widespread, worldwide availability of GPS base-station data on the web today.

SBAS (WAAS/EGNOS/MSAS) didn’t exist, and for real-time corrections and DGPS (beacon) coverage was spotty at best, but real-time commercial DGPS services like OmniSTAR, Landstar, and Satloc were around.

One thing is for sure, no matter what, you have to have some source of corrections to collect GPS data for GIS mapping. It’s commonly referred to as differential GPS correction. Essentially, your GPS receiver needs to reference another GPS receiver (base station) that’s set up on a known position.

Grafnav Post-processing software

There are two primary methods in which to apply a correction to your GPS data: post-processing differential correction and real-time DGPS.

Post-processing

When you’re collecting GPS data that’s going to be post-processed, you need a GPS receiver (and software) that’s going to be able to record satellite observation data. Otherwise, data is collected as one normally would in the field, whether it’s utility poles, manhole covers, road centerlines or polygons of any sort.

The accuracy of the GPS data while you’re in the field is autonomous GPS, so it could be several meters or even ten meters or more. You can’t use this type of method for navigating to a point with any sort of accuracy better than a few meters.

After you’re finished collecting your GPS data for the day, you go back to the office and download your data to your computer. Post-processing requires special software. That software will allow you to search the Internet for the closest GPS base station(s) to use as a source of GPS corrections. In previous years, it was a laborious task to search for GPS base-station data that was recorded the same time as you were in the field (remember UTC vs. local time?). That’s not the case any longer as advanced post-processing software has made this a more automated process. The software will search for the closest base station and automatically select the appropriate files to download.

It takes specialized software and training to utilize post-processing effectively.

Real-time DGPS

This is a method of receiving GPS corrections while you’re in the field. The GPS corrections are applied in real-time so your positioning is accurate. This is useful when you want to navigate to a particular point very accurately. In the 1990s, there were a number of DGPS services, mostly commercial. One would pay a monthly or annual subscription fee to receive the DGPS corrections. During that time, the U.S. Coast Guard started developing a system by which it will install GPS base stations near the major U.S. waterways (coastlines and major rivers). It set up large towers that would broadcast the corrections via 300 kHz radio. Most importantly, it broadcast the corrections free of charge. One only needed a “beacon receiver” to receive the corrections. The system didn’t cover the entire U.S., but it opened the eyes as to what was possible in terms of a regionwide, or nationwide, DGPS network of base stations.

The U.S. Coast Guard concept is still used today in more than 40 countries for DGPS marine navigation. The same GPS correction signal is also used by many people using GPS for mapping.

Around the same time, the Federal Aviation Administration (FAA) began developing a system to improve GPS integrity and accuracy. They called it WAAS (Wide Area Augmentation System). It was the first SBAS in the world and, upon being declared operational in 2003, is in use by thousands of people for GPS mapping. SBAS is a regional system. WAAS only covers North America (U.S., Canada, and Mexico). It has spawned a number of similar and compatible systems such as EGNOS in Western Europe and MSAS in Asia with GAGAN under development in India.

There are several advantages and disadvantages to both post-processing and real-time DGPS for GPS mapping. The primary advantage of post-processing is that you don’t have to worry about a wireless data connection in the field. The primary advantage of real-time DGPS is that you get much better accuracy in the field. There are many other factors you should consider when deciding which method to use.

In fact, I think it’s an interesting enough topic that I’m conducting a webinar later this month that will address both of these methods. I’ve invited Dr. Michael Whitehead to join me. He’s the head technology guy at Hemisphere GPS and has worked extensively developing high performance GPS receivers. He was also the chief architect at Satloc back in the late ’90s.

Tens of thousands of users around the world utilize GPS/GNSS receivers for mapping, surveying and navigating. Since autonomous GPS/GNSS typically does not provide the needed accuracy, users must rely on a source of GPS/GNSS corrections. There are three sources of GPS/GNSS corrections available to users who desire reliable GPS/GNSS accuracy in the sub-meter to three meter range: SBAS, DGPS and post-processing. Dr. Michael Whitehead, Chief Scientist at Hemisphere GPS, will join me in presenting a background on the three technologies as well as the strengths and weaknesses of each. I’ve known Mike for a number of years. He was an early innovator in the development of SBAS technology at Satloc as well as SBAS and DGPS receiver technology at Hemisphere GPS. He is one of the leading GNSS engineers in the world. I’m particularly excited about this event and promise a lively discussion that’s full of useful information, data and concepts that anyone using or considering using GPS/GNSS for mapping, surveying or navigating will find useful.

Eric Gakstatter has been involved in the GPS/GNSS industry for more than 20 years. For 10 years, he held several product management positions in the GPS/GNSS industry, managing the development of several medium- and high-precision GNSS products along with associated data-collection and post-processing software.
Since 2000, he's been a power user of GPS/GNSS technology as well as consulted with capital management companies; federal, state and local government agencies; and private companies on the application and/or development of GPS technology. Since 2006, he's been a contributing editor to GPS World magazine,writing a monthly newsletter on high-precision GPS/GNSS
technology. He is also editor of Geospatial Solutions, a weekly newsletter focused on geospatial technologies.

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